The rf performance of 1 x 200 mum(2) AlGaN/GaN MOS-HEMTs with Sc2O3 used as both the gate dielectric and as a surface passivation layer is reported. A maximum f(T) of similar to11 GHz and f(MAX) of 19 GHz were obtained. The equivalent device parameters were extracted by fitting this data to obtain the transconductance, drain resistance, drain-source resistance, transfer time and gate-drain and gate-source capacitance as a function of gate voltage. The transfer time is in the order 0.5-1 ps and decreases with increasing gate voltage. (C) 2003 Elsevier Ltd. All rights reserved.